Standardization of Radioactive Sources -...

STANDARDIZATION OF RADIOACTIVE SOURCES1,~

BY GEORGE G. MANO¥Atomic Energy Commission, Washington, D. C.

INTRODUCTION

Primary standards of radioactivity are in some ways, analogous to fixedpoints in the temperature scale in that each can be used for comparison with"unknowns" not too far removed from the standard. Over a thousand radio-active species are known to exist, but only a comparatively few satisfy suchbasic requirements for primary standards as: known decay scheme; appro-priate half life and disintegration energy; radiochemical and radioisotopicpurity; and ease of preparation and availability.

Whereas it had originally been considered that a relatively few standardscovering the range of ~3- and T-energies of interest would suffice, the thoughthas been recently expressed that standards may be required for many addi-tional radionuclides. For example, discrepancies in measurements using end-window counters may arise in extrapolating to "zero total absorber" becauseof the difference in the energy distribution of the /~-ray spectra of twoisotopes that have approximately the same maximum fl-energy. Similarly,although a given/~-~/-standard may be suitable for determining quantitativelythe activity of a second radionuclide by comparison of respective beta activi-ties, it may be totally inapplicable, for example, for use in correspondingmeasurements involving comparative ~,-ray counting.

Where chemical methods are involved in the assay, interference has beenfound to occur from analogous compounds of the same element (e.g.,Na2HP3~O4 and Na2HP3~O3), as well as from different radio-species of thesame element in a given compound (e.g., Na~HP~O4 and Na2HP~O4).

Radium was formerly unique in being the only radlonucllde for whichthe establishment of radiochemical purity was equivalent to primary stand-ardization. The curie was officially redefined, however, as reported by Evans(lb) and Paneth (2) at the July, 1950, meeting of the Joint Commission Standards, Units, and Constants of Radioactivity as "the quantity of anyradioactive nuclide in which the number of disintegrations per second is3.700X10~°.’’ This new definition divorces the curie unit from a specialassociation with radon and radium. The use of the term "rutherford" todenote 106 disintegrations per second was abandoned in 1950.

The progress in the acceptance of definitions for radioactivity units andstandards has been reported by Evans (la, lb). The present survey of theproblems involved in the preparation and certification of primary standards

1 The survey of the literature pertaining to this review was concluded on March 1,

1954.* The following abbreviations are used: MIT for Massachusetts Institute of

Technology; N BS for National Bureau of Standards; ORNL for Oak Ridge NationalLaboratory; AECL for Atomic Energy of Canada, Limited.

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http://www.annualreviews.org/aronline

52 MANOV

of radioactivity indicates major progress in the determination of the absolutedisintegration rates of f~- and of/~-~,-emitters. The situation is much improvedover the period 1947-49, and the availability of standards within the UnitedStates such as C14, Co6°, I TM, P~, Ra~, Pb~° (Radium D-E), ~° and T1~°4

is one measure of the advances made.Assembly by the National Bureau of Standards (3a) of published infor-

mation on the decay scheme, half llfe and energy distribution of many radio-nuclides has served as a spur for additional research in these fields. Promptpublication of such summaries is now a routine matter (3b), and some the information is available on punched cards (4).

.ADVANCE IN TECHNIQUES OF MEASUREMENTS

DETERMIN’ATION OF THE HALF LIFE OF RADIONUCLIDES

The determination of the half life of a nuclide by any method at constant,but unknown, e~ciency is essentially a straightforward procedure. If R~and R~ are the observed counting rates obtained with a given detector during

a time interval, t, the half life, T~ can be calculated:

T~ = 0.693t/[2.3 1OglD (R1/R2)] 1.

The method is best adapted to those nuclides for which the initial and finalmeasurements can be obtained in a reasonable length of time.

Half lives have been measured to four significant figures by use of a dif-ferential ionization chamber and a vibrating reed amplifier with feedback(5, 6). With this device, it has been possible to obtain significant measure-ments over a period of 12 half lives with some short-lived radionuclides. Anautomatic recording electroscope has been constructed (7) to determinehalf lives of the order of a few minutes with an accuracy of +0.02 min. If anuclide has a long life (such as C~*), the half lives can be calulated from theexpression

T~ -- 0.693N/(-dN/dt), 2.

where N is the total number of radioactive atoms present (determined bymass-spectrographic analysis and the weight of the sample present) and-dN/dt is the measured rate of disintegration. Calorimetric methods havebeen used to determine directly the half life of radionuclides such as Pa~ (8).

DETERMINATION OF THE RATE OF DISINTEGRATION

Measurements of the rate of disintegration can be made by: (a) coinci-dence counting (either/~-~ or 7-5’); (b) Geiger-Mueller (G-M) or other countersa under conditions of known geometry; and (c) calorimetry. Coinci-dence counting (11 to 15) involves the use of two counters: one for detecting

~ The development of scintillation counters and the advantages thereto havebeen reported in several symposia (9) as well as in the present volume, page 111.Halogen-quenched G-M counters have been developed (10) which are said to have very long operating llfe and which use a visibly transparent, nonmetallic electricallyconducting film as a cathode.


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STANDARDIZATION OF RADIOACTIVE SOURCES 53

the 0- (or 7-) radiation, and the other for detecting the (second) 3,-radiation.Certain conditions must be fulfilled and various small corrections to the ob-served counting rates must be applied. If N~ and N2 are the counting ratesof the ~ (or ~/) and of the 3"-counters, respectively, and N~ is the coincidencerate, the absolute rate of disintegration, -dN/dt, of the sample is given by

-dN/dt ~ N~iV2/N~. 3.

This method can be applied to the determination of the activity of long-livedas well as moderately short-lived nuclides.

The technique of "known geometry" with end-window counting, forexample, involves measuring the rate of decay of a sample under conditionsin which the counter "sees" a known solid angle a~ a known distance fromthe source. In principle the method is simple but corrections must be applied(16) for resolution or dead time losses; for baekseattering from the support;for sidescattering from the walls of the shield enclosing the sample, thesupport and the counter; for absorption by the window of the counter, etc.

The method of end-window counting is applicable preferably to 0-emitterswith moderate or long half lives. A variation of the method of "knowngeometry" is that in which the sample to be measured is completely insidethe counter or may even be part of the filling gas. This type of counter hasbeen successfully used in measurements involving C~4 and H3.

The rate of charge loss has been measured in isolated systems containinga radionuclide by using a Lindemann electrometer and a compensatingcondenser (17). The method has been applied to p3~ and ~.

Internal counters with 47r geometry suggested in 1947 by Borkowski (18)have been refined and used extensively (19a, 19b) for absolute 0-measure-ments. Proportional 4~r counters have been widely used in Canada (20),England (21, 22, 23) and in Germany (24, 25) for standardization of high and low-energy 0-emitters.

Panel discussions (16) on the techniques of absolute 0-counting by theknown geometry method have brought out a number of small, hitherto un-suspected sources of error regarding the effects of backscatterlng from sup-ports, self-absorption, etc. New measurements have been made of the back-scattering of positrons and electrons from various kinds of supportingmaterials (26, 27). The method of Marinelli (28) can be used for standardiza-tion if the decay scheme has been established. The rate of disintegration ofa radionuclide can also be determined (29) if the average energy per disinte-gration, ~2, is known.

Calorimetric methods for the determination of activity involve measure-ment of the rate of heat evolved, dQ/dt, by absorption of the energy emittedduring the decay of the radionuclide. The method involves the usual thermaland other corrections applicable to calorimetry and is used preferably for0-emitters or for other isotopes where the radiation to be measured can becompletely absorbed in the calorimeter. From the expression

dO_/at = ~ aN/dr 4.


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54 MANOV

either -dN/dt or E~, the average energy per fl-disintegratlon, can be com-puted if one of them is known.

The availability of intense, high specific-activlty sources and the de-velopment of suitable calorimeters (8, 30) operating at liquid air tempera-tures has enabled estimation of the half lives of isotopes such as C14.

The consensus presently seems to be (31c) that more reliable measure-ments can be made by internal 4~r and by B-3’ (or 3’-3’) coincidence countingmethods rather than by the method of known geometry. The method usingthe rate of charge loss (17) probably warrants further investigation.

DECAY SCHEMES

A clearer understanding of the techniques involved and of the errorsinherent in coincidence counting (11, 12), the method of "delayed colnci-dence" counting (32), the development (33, 34) of amplifiers with very shortresolving times (10-9 sec.), and the virtual elimination of spurious coinci-dences have permitted closer examination of many complex decay schemes.

A/3-/3 (conversion electron) coincidence spectrometer has been devised(35) in which a Geiger-Mueller counter is placed at the "exit" end of thespectrometer and a scintillation counter is placed near the source. Coinci-dences are measured between the two counters corresponding to a selectedelectron "llne" of known energy and all other #-radiation. The arrangementis said to be satisfactory for establishing decay schemes by B-B (conversionelectron) coincidences.

A new method has been devised (36) for the absolute determination B-ray energies in which a magnetic electron lens, working at unit modifica-tion, uses the image-rotation property to produce two lines by reversal of thecurrent through the lens winding. The specific momentum, Hp, of the focusedelectrons depends, among other factors, on the angle between the two lines.

Internal conversion coefficients have been measured by 3,-x-ray coinci-dence counting (37, 38) as well as by the double-coil, thin lens, magneticB-ray spectrometer (39). A revised table of K-shell internal-conversion

coefficients is available (40).

SPECTROMETRY

Deutsch (41) has made a critical survey of the techniques of B- and

3’-spectroscopy. One paragraph in particular is worth quoting:

The aim of very high resolution spectrometry in the establishment of decay schemesis twofold: it permits the detection of lines which could othcrwlsc not bc resolvedeither from the continuum or from other neighbouring lines. In some of the morecomplicated decays the number of identified lines is almost proportional to the resolu-tion used. In addition the use of precision energy measurements permits the applica-tion of the combination principle to the location of levels. For example in the decayof I TM there are six known ~-ray energies. If these are determined with an uncertaintyof 1.5 per cent there appear seven combinations which might be interpreted as al-ternate decays involving the same initial and final states. With an accuracy of 0.5per cent four of these remain. In fact, coincidence measurements have shown thatonly one of these is real, the other accidental.


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An automatic count and control mechanism has been devised (42) whichstepwise sweeps the E-spectrum automatically and counts the number ofimpulses received as a function of the focusing magnetic current. The deviceis said to duplicate with precision the manual control of a/~-ray spectrometer.

Crystal spectrometers (43) have been developed for the determinationof 3,-ray energies to four or more significant figures. Scintillation spectrome-ters and counters for the measurement of the energy distribution and thehalf life (1 to 30 sec.) of/~-ray emitters have been described (44).

A new type of scintillation spectrometer has been devised (45) in whichincident photons are detected in a primary detecting NaI(T1) crystal; theradiations not completely absorbed in the primary crystal are absorbed in asurrounding cluster of NaI(Tl) crystals which act as an anticoincldencecircuit. The arrangement is said to yield the 3,-ray spectrum directly withoutany interpretation of results which might become laborious, particularly ifthe incident "r-radiation has a complex spectrum. The apparatus has beenused to study CsTM, Na~2, and other radlonuclides.

Pair spectrometers have been constructed (46, 47) in which rotation the image in a short magnetic lens is used to separate negatrons and posi-trons by putting the pair converter a little to one side of the optical axis ofthe lens, from which it is possible to obtain coincidence "lines." With thisapparatus the 2.62 Mev 0,-ray of ThC" (TI ~°8) and the 2.76 Mev 3’-ray ofNa24 have been studied. A recent study has been made of the shapes of/~-spectra (48), and tables for the analysis of/~-spectra are available (49).

MASS ABSORPTION COEFFICIENTS

New measurements have been reported for ~’-ray absorption coefficients(50 to 53) over a range of energies from 0.1 to 6 Mev (Co6°, I TM, and others)in absorbers of atomic number from 13 to 82. The calculated values are ingood agreement with those measured experimentally. This information isuseful in correcting for the attenuation of the ~’-radiatlon passing throughthe walls of the G-M counter or ion chamber.

STATUSOF INDIVIDUAL RADIONUCLIDES

For the purpose of this presentation, the status of individual radio-nuclides is divided into three categories: (a) nuclides for which standards the United States are available or are in preparation; (b) nuclides for whichno standards are available, but for which considerable information on nuclearproperties exists; and (c) nuclides for which standards would be desirablebut for which considerably more informatii~n of various kinds is needed.

In each category, the information listed includes (when available): themethod of production; method of obtaining radiochemical purity; the halflife; the decay scheme; intercomparison of the activity between differentlaboratories; and the availability of standard samples. The nuclides are listedalphabetically within each category.


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56 MANOV

AVAILABLE STANDARDS

Summaries are presented for the following radioncclides: C14, Co60, p31,P3~, Ra~, Pb-Bi~° (Radium D-E), ~°, and TI~°4.

Carbon-14~Most of the C1~ produced in the United States is now ob-tained by neutron irradiation of beryllium nltrlde, Be3N~. The C1~ is sepa-rated as active CO2 and precipitated as BaCOn. Specific activities of 250 to1500 mc./gm, of BaCO3 are routinely available (54).

Labeled organic compounds are not suitable as standards for C~. Skraba(55) has found that CI~HaOH, for example, decomposes under self-lrradlationto methane, ethylene glycol, glycerol, and other compounds to an extentsufficient to render it useless for primary standardization.

Full agreement has not yet been reached for the value of the half life ofC1~. Because of the long half life of this radionuclide a direct measurementis not possible, and values ranging from a high of 7200 to a low of 4700 yearshave been obtained by indirect methods. Two general methods have beenused: (a) end-window counting of solid barium carbonate; and (b) countingof gaseous C1~ either as carbon dioxide or as methane in which the radioactivecarbon forms a portion of the filling gas of the counter. Generally speaking,gas-phase counting has proven to be more reliable than the counting of solidbarium carbonate.

Anderson (56) reviewed the techniques for detecting and measuring ~,

and Broda and co-workers (57) claim to have found it possible to measureas little as 2X10-is curies (4 disintegrations/rain.) of t4 using CO~-CS2filled counters.

Hawkings et al. (58) reported a half life of 6360 +200 years using a proce-dure involving a mass-spectrographic analysis of the active CO~ and a pair ofcompensated COs-CS~ filled (59, 60) internal gas counters of varying diame-ters. Jones (61), using alcohol-argon as a filling gas in an internal G-Mcounter, obtained 5589 + 75 yr. for the half llfe of C~. Libby and co-workers(62, 63) also used alcohol-argon as the filling gas and employed counters various ratios of length to diameter. The value for the half life first reported,5720 +20 years (62), was lowered to 5580 +45 years (63) after further rections were made for the end- and the wall-effects of the counters. It wasassumed that the counting efficiency was 100 per cent.

In 1949 the National Bureau of Standards undertook a cooperative ven-ture to determine the half life of Cu and to prepare standard samples of C~4

for distribution. A sample of specially purified BaCO~ treated to removesilica and other inert impurities was used as the starting point. Details ofthe NBS measurements have been reported by Manov & Curtiss (64),

The COs from a portion of this active barium carbonate was generated byheating an intimate mixture of BaCO~ with PbCI~ at first to 500° C. and thento 1100° C. The evolved COs was mixed thoroughly by repeated condensationwith liquid air and evaporation into a small, closed flask and allowed to standfor several hours. Samples of this COs were sent to sew~ral cooperating labo-ratories for mass spectrographic analysis.


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STANDARDIZATION OF RADIOACTIVE SOURCES $7

An accurately weighed portion of the same active barium carbonate wasmixed with a known weight of inert barium carbonate, the CO2 evolved withperchloric acid and absorbed in an excess of sodium-hydroxide--sodium

carbonate solution. The solution was diluted to a known volume, and sam-ples were distributed to the participating laboratories for counting.

It is to be noted that the problem of determining the half life of Ct4 hadbeen divided into two independent parts: (g) a collaborative determination

of the isotopic ratio from which N in equation 2 could be calculated; and(b) a collaborative determination of -dN/dt. The results were reported inFebruary, 1950 (31a, 31b) and later (65) in more detail.

In each case, gas-phase counting was performed with a compensated pairof CO2-CS2 filled counters. It is known (59, 60) that the resolving time the CO2-CS~ counters is of the order of 1000 #sec., and at counting rates suf-ficiently high (50 to i00 counts/sec.) to obtain results of statistical signif-icance in a reasonable length of time, the resolutlon-time losses are of theorder of several per cent. (64)

The counting rates obtained by Chalk River and by MIT~ were, at thetime, provisional values. As an interim value pending further work, it wastherefore suggested (31a) that the average isotopic ratio of the active bariumcarbonate used in the cooperative investigation be considered 3.88 per cent,the activity of the ampoules be tentatively assigned 1280 disintegra-tions/sec./ml., and the half life of C14 be estimated at 5400 years, with anuncertainty of perhaps + 200 years.

Subsequent to this suggestion, two additional values for the half lifeusing other samples of BaCO~ have been reported by Bernstein & Ballentine(66a) in which it appears that gas-phase counting in the proportional regionand in the Geiger-Mueller (G-M) region yield 5513 and 6360 years respec-

tively. This discrepancy appears to have been partially resolved in a laterpaper (66b) in which the hypothesis was advanced that "over-quenching"of the G-M counter may have occurred which led to a significant loss (13.9per cent) in the counting rate. Provided this observation and the explanationare correct, it may be appropriate to reduce the value for the half life ob-tained (66a) in the G-M region to 6360/1.139 or 5580 years. Gas-phasecounting would therefore seem to lead to estimates of 5513 and 5580 yearsfor the two sets of measurements. The agreement, however, may be fortui-tous.

Evidently the last word on the half life of C14 has not yet been said andmore experimental work needs be done. There seem to be one set of measure-ments in the range .~400 to 5600 years and another 6000 to 6200 years.

The maximum energy for the ~-spectrum of C1~ is 155 _1 kev (67), andthe decay scheme involves only a simple ~-emission. Standard samples ofCI~ are available from the National Bureau of Standards.

Cobalt-gO.~Co~ is generally prepared by neutron absorption in Co~;sample preparation consists of solution in hydrochloric or other acids (withor without added cobalt carrier) and dilution to the appropriate activity.

The value for the half life of Co~° is approaching agreement. Recent de.-


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58 MANOV

terminations yield 5.26 _+0.17, 5.27 +0.07, 5.25 +0.21, and 5.21 +0.04 years(6, 68, 69, 70), with an average of 5.25 years.4

The decay scheme involves a simple fl-decay with Emax°8 = 0.306_+ 0.005Mev (72), followed by two ~’-rays in cascade whose energies (43) are 1.1715-+0.0010 and 1.331 -+0.0010 Mev, respectively.

The Bureau of Standards has prepared a set of ampoules containing Co6~

which are dilute with respect to total solids (i.e., approximately 1 #g./ml.)and has measured the activity of these ampoules by fl-’y-coincidence, 4~r, andother counting methods. Preliminary results were reported by Mann &Seliger (27) and by Putnam (21). The former (27) studied two variations the 4~r fl-counter (conducting film and mirror image) and two coincidence-counting methods (fl-~" and 7-7). On the same sample of s°, th e re lative

results obtained (assigning arbitrarily a value of unity to the first method)were 1.000, 0.996, 1.003, and 1.006.

Iodine-131.--I I~I is presently separated from fission products by oxidationto elemental iodine and volatilization from an aqueous solution. The radio-nuclide is distributed in the form o~ a weakly basic iodide solution containing

a trace of NaHSO~ (54).The half life has been reported by Sreb (73) to be 8.1409 -+0.0062 days’,

8.04 .+_0.04 days by Sinclair & Halloway (69); 8.06 -+0.02 days by Lockett Thomas (71); 8.075 _+0.022 days (measured over five half lives), by Sellger,Cavallo & Culpepper (74); and 8.05+0.01 days by Hawkings and hisco-workers (75). The average value, neglecting (73) is 8.06 days.

Decay schemes for I ~t have recently been reported by Bell, Cassidy &Kelley (76) and Cork and co-workers (77). Both schemes recognize the istence of the 12 day metastable state of Xe~ reported by Zeldes, Brosi &Ketelle (78), but associate it with different/~-energies. Additional complexi-ties in the decay scheme have since been discovered. Verster (79), Ketelle(80), Cavanagh (81), and Bell (82) and Kurbatov (83) have confirmed existence of two additional fl-energles, 250 kev (2 per cent), and 807 key(1 per cent). Rose (84) has measured internal conversion electrons but

parently did not observe the 250 key/~-energy. Bell (85) has recently founda previously unknown "y-energy of about 0.5 Mev that does not fit into thepresently accepted decay schemes. Caswell (29) has found the average energy

of the fl-radiatlon to be 0.189 _+0.008 Mev.A number of methods are in use for the standardization of I~; known

geometry, coincidence counting, 4~r counting, and ionization methods prevail(16). The status of the intercomparison of iodine standards is much morepromising than it was six years ago.

The National Bureau of Standards since 1950 has been furnishing on aregular schedule 3-ml. ampoules of 11~ (and of P~) with a known activity( + 3 per cent) in terms of disintegrations per second per milliliter of solution.The activity is of the order of 100,000 dlslntegrations/sec./ml., with higher

* It is understood that the value of 4.95_+0.04 years (71) has been withdrawn(3~c).


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activities occasionally available by special arrangement. Provision is madefor pH control, the samples are low in total solids (approximately 10 #g./ml.),and instructions for use accompany each sample. 4¢ proportional countingis used for standardization (19a). The presence of the metastable, 12 dayXeTM introduces an error which can be neglected in end-window fl-countingor in ionization chamber measurements where an overall accuracy of +5per cent is sufficient. Approximately 200 ampoules are distributed at six-

month intervals to users within the United States, Australia, Canada andGreat Britain, and frequent inter-checks between NBS,20RNL,~ and otherstandardizing laboratories serve to maintain the requisite accuracy instandardization.

An international comparison tabulated by Seliger (19b) for samplesdistributed by the NBS shows the following relative values (the NBS valueis arbitrarily designated as unity) : NBS, United States, 4~r fl (!.000) ; AECL,2

Canada, 4~r (1.022) ; and two values from Hammersmlth Hospital, England,

absolute 7-ion chamber (0.997) and 4~r G-M (1.030).Samples of I ~zt are being exchanged in an attempt to establish an inter-

national standard. The present agreement appears to be of the order of 4-2per cent.

-Phosphorus-32.--Neutron irradiation of sulfur, followed by chemicaloxidation and separation, is used to obtain P3~ (54). If sulfur-containingcompounds are irradiated to form phosphorus, the chemistry of separationis rather involved because of the different radiochemlcal forms in which thephosphorus may be found. For example, if calcium sulfate is irradiated,metaphosphates (PO~-), ortho-phosphates (PO~--), pyro-phosphates(P207----) and phosphites (HPO~--) are formed. Failure to recognize chemistry involved may lead to abnormal tracer behavior (69, 86).

New measurements have been reported for the half life of P~. Using acalorimetric method, Bayly (8) obtained 14.30_+0.05 days; Lockett Thomas (71) used a quartz fibre electroscope and obtained 14.50 _+0.04 daysover a period of five half lives. Neither worker, however, was aware of thepossible presence of P~8 (see below). The maximum ~3-energy for P~ has beenfound to be 1.704 +0.008 Mev (87a).

The existence Of P~ in reactor-produced P~ has been reported by Jensen(87a, 88) and confirmed by Sheline (89) and others. The half llfe is 2.44 ___0.2days, and the maximum fl-energy is 0.249 -+0.002 (87b).

Collaborative determinations of the absolute strength of P~2 sampleshave been carried out for a number of years between several standardizinglaboratories, and the results have recently been summarized by Seliger(19b). The laboratories reporting, the methods used and the relative values

obtained (19b) are as follows: United States NBS, 4~r fl-counter (1.000);ORNL, defined solid angle (0.986); Columbia University, parallel platecounter (1.006); AECL-Canada, 4~r E-counter (1.002); Hammersmith pital, England, absolute fl-ion chamber (0.971).

Ampoules containing known amounts of P~ have been available since


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1950 from the Natlonal Bureau of Standards. These ampoules contain ap-proximately 100,000 disintegratlons/sec./ml, of P~ with 0.001 M H~P04added as carrier. The effect of p~3 on the measurements is considered to benegligible if the time interval between NBS standardization and use involvesless than two half lives. 4~r proportional counting is used (19b). The avail-ability and the method of distrlbution~ for p~2 and I TM are identical.

Ragium-Zg~.--Radium standards have been available for many years,but minor uncertainties still persist in determining the activity of one gramof radium. Careful measurements of the specific activity of radium were madeby Kohman (91) in 1947, yielding (3.608 4-0.028) l° a-dlsintegrations persecond per gram of radium, and a half life of 1620 + 13 years. One curie (ofradium or of any other nuclide) was redefined in 1950 to be equivalent to3.700X10~ disintegrations per second (lb, 2).

Lead-Bismuth-210 (Ra D-E).--Pb21° (RaD) is obtained by chemicalseparation from the equilibrium radium mixture, and an organic reagent,dithizone, is useful (92) for this purpose. Electroplating under carefully con-trolled conditions (93) is also employed to effect the separation. ~t° (RUE)is obtained as the daughter product of Pb~°, and wdues of 5.02 4-0.02 and4.989 4-0.013 days have been reported (71, 94) for the half life.

The original method for preparing standards of RuE was the subject ofdiscussion between the National Bureau of Standards and the NationalResearch Council, and the method decided upon at the time, early in 1946,was a compromise between (a) accurate standards desirable for the future,and (b) the immediate need for a fairly large number of reproducible stand-ards with a somewhat lesser degree of accuracy. The NBS standards arecomposed of a circular deposit of active and inert PbO~ weighing approxi-mately one milligram; the material is deposited electrolytically in the centerof a palladium-faced silver disk approximately one: inch in diameter and1/16-inch in thickness. Calibration was initially made by assaying accuratelya radium ore for activity, extracting the total lead therefrom, and then elec-trolytically depositing a portion of the lead as lead peroxide. Several hundredsuch standards have been prepared and distributed.

In 1950, Burtt (95) and others found that there was a forward scatteringof the 0-particles from RaE because of the finite thickness of the PbO~ de-posit. The scattering amounted to approximately 7 per cent under the par-ticular conditions of geometry, backscatter, sidescatter, etc., used by him.Seliger (26) has since made additional measurements of the/~-particle back-scattering from various sources.

These refinements in measurement point up a fact that has long been

~ Ten microeurie quantities of radionuclides with a half life of less than 30 daysand 1 ~c. quantities with a half llfe over 30 days may be interchanged betweenlaboratories without prior approval by the U. S. Atomic Energy Commission (90).This action has served to stimulate intercomparisons not only of mater/als proposedas standards, but of various intermediate preparations. It applies, however, only totransfers within the United States.


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recognized; i.e., that measurements made by comparison of the E-activitiesbetween a standard of a given radionuelide and the "unknown" should bemade under identical conditions of geometry, thickness of deposit, back-scattering, and that the energies of the standard and of the "unknown" andthe shapes of their E-spectra should be as nearly alike as possible.

An independent calibration of the strength of the RaE sources can beobtained by measuring the growth of the a-activity from RaF. Such measure-ments have been made by Novey (96) and indicate that if proper correctionsare applied, the agreement between 8- and a-counting is within 2 per cent.National Bureau of Standards Pb21°-Bi~1° standards are available in activitiesapproximating 100, 200, 500, and 1000 disintegrations./sec., and have beenwidely used in this country and abroad. The international standardizationof Pb21°-Bi21° has not yet been attempted.

Strontium-90.--Sr9°, obtained by separation from fission products, is apure E-ray emitter which decays to yg0 and then to stable Zr9°. A value of19.97 +0.3 years (97) has been reported by Powers and Voigt for the halflife, measurements being made over a period of seven years. The heretoforeaccepted figure is 25 years.

The maximum E-energies of Sr9° and yg0, according to Cohn (98), are0.525 and 2.24 Mev respectively; for Yoo, Moreau & Corba (99) found2.270 +0.02 Mev. Similar measurements have been made by Shepherd (100)on yo0. The average E-energy of y~0 is 0.895 +0.035 Mev (29).

The National Bureau of Standards has available for distribution standardsamples of Sr~° in equilibrium with yg0. The activity, approximately 10,000dlsintegrations/sec./ml, of solution, is standardized by 2~r/~-counting, withan accuracy of + 1.5 per cent.

Thallium-204.--T1TM is prepared by neutron irradiation of T1~°~ and ismade available as an aqueous solution of the nitrate. Various difficultieshave been encountered in the preparation of samples with reproduciblecounting rates. T1~°4 has chemical oxidation states of +1 and +3 in aqueoussolution, and it has been reported by Peacock (31b) that electroplating is notsatisfactory as a method of separation from other electrolytes. Thallousnitrate solutions in dilute nitric acid have been found by Reynolds (101) be more stable than neutral or alkaline solutions of thalllc ion or halidesolutions of thallous ion.

Values for the maximum E-energy of T1TM have been reported to be 0.7654-0.010 Mev by Lidofsky, Macklin & Wu (102) and 0.760 4-0.01 Mev by derMateosian & Smith (103). The Kurie plot is linear down to 0.150 Mev (102),

A value of 4.0 4-0.1 years for the half life of T1~°~ has been obtained byHarbottle (104), and recently Lockett & Thomas (71) found 2.71 +_0.05 yearsusing a quartz fibre electrometer with measurements extending over a periodof eight months. Two older values (3) of 2.7 and 3.5 years have also beenreported.

Reynolds (105) has recently published the results of a limited intercom-parison (six laboratories) of a solution of ~°* whose activity us ing different


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counting methods, was approximately 1 ~(10-8 #c./ml. The results appearpromising, with an average deviation of + 10 per cent. Further work is underway.

Standard samples of T12°4 are available from the National Bureau ofStandards in the form of 3-ml. ampoules with an activity of approximately10,000 disintegrations/sec./ml. Standardization is performed by 2~r ~-counting with an accuracy between -t-4 per cent and --2 per cent.

DESIRABLE STANDARDS; SOME NUCLEAR INFORMATION AVAILABLE

Summaries are presented for the following radionuclides: Au128, Fe5~,

Fe~, Na24, and Ha.

Gold-198.--The half life of Au198 has been found to be: 2.69 +0.01 (5),2.73 +0.01 (69), 2.697 +0.003 (71) and 2.74+0.04 (72) days. The scheme has been studied recently by a number of workers (72, 106 to 109).The major portion (98 per cent) of the gold decays by a simple/~-emissionfor which values of 0.97 +0.01 and 0.958 Mev have been found (42, 72).The single "y-ray energy has been measured yielding a value of 0.411770+0.000036 and 0.41173 +0.00007 Mev (110, 111). The minor portion decayswith a fl-energy of 0.290 Mev and a "r-energy of 1.09 Mev. The Auger spec-trum indicates that I~ capture to Pt108 cannot occur in more than 0.5 per centof the disintegrations (112). The work of Brosi (107) confirms that Cavanagh (106). It has been determined (34) that the half life of excited(411 key) ~gs resulting fr om the/~-decay ofAul ~s is less than 3 X10-usec.Hill & Mihelich (109) observed two weak "y-rays of 159 and 209 key whichhave been attributed to Au~9~ formed by the successive reactions: AuTM

(n, ~/) AutOS; ~s (n, 3~) Au1~. The half li fe for Au~ was observed tobe 3.3days, the maximum fl-energy is 0.32 Mev (3) and the calculated thermalneutron cross section of Au~os is estimated to be about 3.5 X 104 barns. Therelative proportions of Au~98/Au~ were estlmated~ to be 95/1 for a sampleirradiated for eight days in a flux of 10TM n/cm.2/sec. (109).

Standards for Au~s are needed. Presently, measurements of the strengthof these sources are generally made by comparison with a radium needleusing an ionization chamber counter or electroscope. ORNL assays Au~8

routinely by means of a high pressure ionization chamber calibrated peri-odically by 4~r coincidence counting of the same sample. Because of its com-paratively short half life, it is doubtful that standard samples of Au~ canbe distributed in the usual sense.

From the practical point of view, two courses of nction are open: (a)standardization of a Aul~s source by coincidence counting and simultaneouscomparison with radium in an ionization chamber under rigidly specifiedconditions (the gold might then be certified in terms of radium-equivalents) (b) search for an isotope of reasonably long half llfe that has either the same

6 A private communication from H. H. Seliger of NBS indicates that for an in-finltely long irradiation time, the ratio of AuX~S/Au~#~ should be approximately 1000/1.


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B" or "r-energy as Au19s (Tin17° has been suggested). Secondary standardsmight then be distributed for comparison of "unknowns" with either the/~- or the 3,-radiation, as appropriate.

Iron-55.--Fe5~ is of interest because the decay is thought to proceedalmost entirely by K-capture. Scintillation counting (113) has shown thatthe continuous radiation has an upper limit of 0.205 Mev, and a later value(38) of 0.206 +0.020 Mev has been reported. A new value for the half life,3.0 +0.15 years has been found (114) which is in agreement with an earlierone of 2.94 years (68). Preliminary results on intercomparlson of samples Fe~ reported by Reynolds (101) indicate that an agreement of the order _+ 15 per cent is possible.

Iron-59.--Fe~ is generally produced by cyclotron bombardment. Threevalues for the half life have been reported: 45.1 __.0.2, 47.1 +0.5, and 45.0_+0.2 days (6, 114, 115). The decay scheme has been studied by Rose and

co-workers (116) and by Metzger (117).Sodiurn-g4.--Na~4 is produced by neutron irradiation of stable sodium.

The half llfe of Na~4 has been reported by Sreb (118) to be 15.060 +_0.039hr., by Lockett and Thomas (71) as 14.97 ±0.02 hr. and an earlier value Wilson & Bishop (119) recalculated by Sreb to yield 14.96-1-0.10 hr. Thethree values agree within the limits of error.

Considerable interest has been manifested in the decay scheme of Na~

Turner & Cavanagh (120) have found evidence for a 4.17 Mev B-group andestimate the branching ratio to be 0.003 per cent. Siegbahn & Johansson(46) have used a new type of pair spectrometer in which rotation of the imagein a short magnetic lens is used to separate negatrons from positrons by put-ting the pair-converter a little aside from the optical axis. The simultaneousemission of three particles (a decay B-ray and an electron-positron pair) fromNa~4 has been examined (121) and the value of the pair-conversion co-efficient for the 2.78 Mev 7-ray has been determined to be (7.6 _ 1.9) X -4.

A pair conversion coefficient of (8.25 _ 1.05) × -4 was obtained for positronsstopped in Be. Hedgran and Lind (122) have measured the energy of the3’-rays from Na~4 and report 1.3680 _+0.0010 and 2.7535 ±0.0010 Mev.

Absolute measurements of the activity of Na~4 have been performed byusing cavity-type ionization chambers (22) and have been compared withthe results of coincidence counting (21). It will be difficult to set up standardsfor such a short-lived material.

Tritium.mTwo recent values for the half life of tritium yield 12.46 ___0.1and 12.41 ___0.20 years (123, 124). Recent measurements of the maximumfl-energy by Langer & Moffatt (125) yield 17.95 _+0.10 key, while the averageenergy (123) is 5.69 ±0.04 key. A bibliography of literature on tritium hasbeen compiled (126).

The low-energy of the fl-radiatlon from tritium makes quantitativemeasurements difficult or impractical by ordinary methods such as end-window counting and recourse must be had to gas-phase measurements(119, 127, 128). For samples which are essentially pure tritium, the method


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64 MANOV

of gas-density (123) can be used and it is claimed that an accuracy of ±0.05per cent can be attained within the range of 50-100 per cent T2. A check onthe composition of tritium-deuterium-hydrogen mlxt:~res is obtained bymass-spectrographlc analysis. (The tritium can be sto~’ed on spongy Zr orPd, and released by gentle heating.)

For lower concentrations of tritium, internal gas-counting can be used.The counters are of brass with flat end-plates similar to those used byLibby (62) with alcohol-argon in the ratio of 1 to 9 at a total pressure of cm. as the filling gas. The tritium is quantitatively diluted with ordinaryhydrogen to give a reasonable counting rate in the G-M tube. A countingefficiency of 97 per cent is assumed, and the length of the plateau is approxi-mately 200 volts with a 1000 volt threshold and a slope of approximately 3per cent per 100 volts. Backgrounds of approximately 200 c/rain, on acounter of 1~(12" are obtained. Unfortunately, some memory effects areassociated with this type of counter, and the background has been found toincrease approximately 200 counts/mln, for each filling. The counters can,however, be cleaned thoroughly by rinsing with dilute aqueous ammoniaand washing with distilled water. Robinson (127) has used a mixture tritium and methane as a filling gas. The advantage claimed is that the re-solving time is very short, that counting can proceed at a h~gh rate of speedwithout undue coincidence losses, and that the memory effects are smaller.

It should also be possible to use a vibrating-reed electrometer and an ionchamber for routine measurements. Such a chamber could be calibrated athigh concentrations of tritium by means of the gas-density balance and atlow concentration by G-M gas-phase counting.

It was recently proposed (31b) that standard samples of tritium be pre-pared in the form of tritiated water with an approximate activity of l0G to10~ disintegrations/sec./ml. This work is now under way.

DESIRABLE STANDARDS; LITTL:E NUCLEAR INI~ORMATION AVAILABLI~

Summar[es are presented for the fo]]owlng radlonuc]ides: Crs~ and Znss.

Chromium-51.~A value of 27.75 ±0.3 days for the half life has been re-ported recently by Lyon (129) and the decay scheme which is 90 to 92 percent K capture has been examined more thoroughly (129, 130). Reynolds(i01) has measured the disintegration rate of the same sample of ~ (a) byx-~,-co{ncidence counting and (b) by x-ray counting with a proportionalcounter standardized by means of a source of Mn~ of known disintegrationrate. Results by the two methods agreed within 3 per cent.

Z~n¢-65.--A new measurement (lI5) of the half llfe has ylelded 245.04-0.8 days. A small percentage of the dls[ntegrafions are by pos]tron emis-

sion, the main decay process being by K-capture. Furberg (131) has de-termined the branching ratio K~/(K,~-K~) to be 0.44 +_.0.03, xvhere K~-- num-ber of capture processes leading to the ground state, and I(2 - number leadingto the excited state. Major (132) has found 0.45 for the branching ratio,and Gril~iths (133) by coincidence measurements found that one positronwas emitted for every 65 ~: 5 ~’-rays.


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Mann and co-workers (134) measured the/3-spectrum by means of a thinlens type magnetic spectrometer and found 0.325 +0.002 Mev for thepositron and 1.38 +0.03 Mev for the 7-radiations.

LITERATURE CITED

1. (a) Evans, R. D., Nucleonics, 1(2), 32-43 (1947); (b) Hahn, P. RadioisotopeTherapy (footnote p. 10) (Academic Press, Inc., New York, N. Y., 1931)

2. Paneth, F. A., Nucleonics, 8(5), 38 (1951)3. (a) Way, K., Fano, L., Scott, M. R., Thew, K., and Wood, M., National Bureau

of Standards Circular 499, Suppl., l, 2, 3 (1930)4. Technical Information Service, Atomic Energy Commission, Washington,

D. C.; Radioactive Products Co., 443 West Congress St., Detroit, Michigan5. Silver, L. M., Can. J. Phys., 29, 59 (1951)6. Tubailem, J., Compt. rend., 235, 1360 (1951)7. Bartholomew, R. M., Brown, F., Howell, W. D., Shorey, W. R. J., and Yaffe,

L., Can. Y. Phys., 51, 714-22 (1953)8. Bayly, J. G., Can. J. Research, 28[A], 520-29 (1950)9. Symposium on Scintillation Counting (Washington, D. C., Jan. 26-27, 1954)

10. Clark, L. B., Sr., Rev. Sci. Instr., 24, 641-43 (1953)11. Bleuler, E., and Goldsmith, G. J., Experimental Nucleonics (Reinhardt & Com-

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lS, 1948)14. Dunworth, J. V., Rev. Sci. Instr., 11, 167-80 (1948)15. Mitchell, A. C. G., Rev. Mod. Phys., 20, 296-304 (1948)16. Conference on Absolute Beta Counting (National Research Council, U. S. A.,

Preliminary Report No. 8, October, 1950)17. Clark, R. K., Rev. Sci. Instr., 21,753-59 (1950)18. Borkowski, C. J., Conference on Absolute Beta Counting (National Research

Council, U. S. A., Preliminary Report No. 8, Paper 11, October, 1950, un-published data)

19. (a) Seliger, H. H., and Cavallo, L., Y. Research Natl. Bur. Standards, 47, 41(1951); (b) Seliger, H. H., and Schwebel, A., Nucleonics, 1217], 54-63 (1954)

20. Hawkings, R. C., Merritt, W. F., and Craven, J. H., The Maintenance of Radio-active Standards with the 4-pi Proportional Counter (Symposium held at theNational Physical Laboratory, Teddington, England, May, 1951)

21. Putnam, J. L., Brit. J. Radiol., 23, 46-63 (1950)22. Gray, L. H., Brit. J. Radiol., 22~ 677-97 (1949)23. Smith, D. B., Atomic Energy Research Establishment Report, AERE-I/R-1210

(June 18, 1953)24. Meyer-Schutzmelster, L., and Vincent, D. H., Z. Physik, 154, 9-20 (1952)25. Houtermans, F. G., Meyer-Schutzmeister, L., and Vincent, D. H., Z. Physik,

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66 MANOV

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Research, 27[B], 545 (1949)59. Hawkings, R. C., Hunter, R. F., and Mann, W. B., Can. J. Research, 27[B], 555

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7 Available from Radioisotope Sales Dept., Oak Ridge National Laboratory, PostOffaee Box P, Oak Ridge, Tenn.


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